DNA Methylation and Transposable Genetic Elements

Abstract

The process of DNA methylation has excited the imagination of many investigators since Goldetal. (1963 a, b) demonstrated an enzyme which transferred methyl groups from S-adenosylmethionine (SAM) to DNA. The feasibility of this process had been indicated by the discovery that a similar transfer was catalyzed in vitro by soluble enzymes from Escherichia coli and a variety of cells from higher animals and plants (Boreket al. 1964, and Borek 1963). The appropriate substrate for the in vitro reaction was found by accident when E. coli K 12 W 6 was deprived of an essential nutrient, methionine. Unlike most strains of bacteria which stop growth and RNA synthesis when starved of an essential amino acid, this one continues to produce RNA for a time even though it can no longer methylate the RNA because the precursor SAM requires methionine for its regeneration. Such strains have been called “relaxed” since there is a loss of the normal control of RNA transcription. The RNA isolated from the relaxed strain after methionine starvation was found to readily accept methyl groups when incubated with a soluble cell extract and SAM, that had been labeled in the methyl group derived from the 14C-methionine residue. The small RNA made by E. coli K 12 W 6 in vivo was isolated from the reaction mixture and analyzed for radioactive ribothymidylate. The finding of this methylated uridylate labeled with 14C indicated that the RNA was methylated at the polymer level since no RNA synthesis was occurring in the in vitro reaction.